In open heart surgery, the heart and lungs are bypassed via a prosthetic or extracorporeal blood flow circuit so that the heart may be stopped and isolated for surgical repair. This extracorporeal blood flow circuit, popularly called a heart lung machine, contains the means to remove blood from the venous side of the patient, pump and oxygenate the blood, and return it to the arterial side of the patient. This circuit also has a variety of individual elements which perform certain functions such as heaters/coolers to elevate or decrease the temperature of the blood, components to add blood volume or cellular components, blood filters and so forth. These discrete elements of the circuit is interconnected by plastic tubing which, in turn, are connected to a patient completing the circuit. The means of pumping the blood throughout this bypass circuit has typically consisted of roller pumps. These roller pump work by rotating a mechanical arm which radially impinges and collapses a portion of the plastic tubing, one side of which is confined against a rigid semicircular wall. This action acts to propel the blood through the circuit. The flow rate of the blood is directly proportional to the rotary speed of the pump as this type of pump is a positive displacement device. Pumps of this type have been used for many years and are still the predominant type of blood pumping mechanism in use for open heart surgery.
Centrifugal pumps are small disposable devices and round in planar shape. These pumps typically have two compartments, a magnet compartment and a blood compartment. The magnet compartment is sealed and contains a bearing supported disk which contains a number of radially placed magnets. Coupled to the disk and protruding from the magnet compartment into the blood compartment is a shaft. Seals around the shaft prevent fluid from the blood compartment from entering into the magnet compartment. Attached to the shaft is an impeller which is contained within the blood compartment. Two tubes extend from the housing of the blood compartment.
An inlet tube extends axially from the blood compartment. An outlet tube extends orthogonal from the inlet tube and is located tangentially to the blood compartment. The pump is connected to the circuit by connecting the inlet connector of the pump to the inlet tubing of the prosthetic circuit and connecting the outlet connector to the outlet tubing of the prosthetic circuit. The circuit is then primed with fluid to remove air.
A disc similarly configured with magnets of opposite poles in it facing the magnets of the centrifugal pump is coupled to the shaft of a motor. This motor/disc system is then placed adjacent to the magnet disc of the pump. The magnets of the motor disc will attract the magnets of the pump disc and align themselves to each other. Energizing the motor will cause the motor/disc to rotate which, in turn, will cause the pump disc to rotate identically via the coupled magnets. This rotation also turns the shaft and the impeller of the pump as they are mechanically linked and blood is pumped. The centrifugal pump, unlike the roller pump, is not a positive displacement system, rather the centrifugal pump generates pressure which is converted into blood flow as a function of the downstream head of the system.